This invention relates to a radiation image storage panel using a stimulable phosphor, more particularly to a radiation image storage panel which can be used for a long term.
A radiation image such as X-ray image is frequently used for diagnosis of diseases, etc. For obtaining such X-ray image, there have been used the so-called radiation photograph. These are obtained by irradiating X-ray transmitted through a subject onto a phosphor layer (fluorecent screen) to produce a visible light and passing the visible light to a film using silver salt such as used in taking conventional photographs, and consequently developing the film. However, in recent years, methods of forming images directly from the phosphor layer without use of a film coated with a silver salt have been devised.
As such a method, there is a method for imaging in which a radiation transmitted through a subject is absorbed onto a phosphor, then the phosphor is excited with, for example, light or heat energy to thereby permit the radiation energy accumulated in this phosphor by the above absorption to be radiated as fluorescence, and the fluorescence is detected. Specifically, for example, U.S. Pat. No. 3,859,597 and Japanese Unexamined Patent Publication No. 12144/1980 discloses a radiation image storage method employing a stimulable phosphor with a visible light or infrared ray as a stimulating excitation light. This method employs a radiation image storage panel having a stimulable phosphor layer on a support. In this method, radiation transmitted through a subject fall on the stimulable phosphor layer in the radiation image storage panel to accumulate radiation energy corresponding to the radiation transmission degrees at respective portions of the subject and form a latent image, then the stimulable phosphor layer is scanned with a stimulating excitation light, thereby the radiation energy accumulated at the respective portions is permitted to irradiate to be converted to light, and subsequently an image is obtained according to the optical signals depending on the intensity of the light. The thus obtained final image may be reproduced as a hard copy or reproduced on CRT.
Since the radiation image storage panel used in such a radiation image storage method radiates the accumulated energy by scanning with a excitation light after the accumulation of a radiation image information, it can accumulate again a radiation image after the scanning and can be used repeatedly.
Accordingly, the above-mentioned radiation image storage panel desirably to have a property that it can be used for a long term or used many times repeatedly without deterioration of image quality of the radiation image to be obtained. For the above, the stimulable phosphor layer in the above-mentioned radiation image storage panel is required to be sufficiently protected from physical or chemical stimuli from outside.
Particularly, when the above-mentioned stimulable phosphor layer absorbs moisture, radiation sensitivity of the above-mentioned radiation image storage panel is reduced or a retention time of the accumulated energy before irradiation of the excitation light is shortened to give rise to deterioration of the image quality of the obtained radiation image. These changes are reversible, namely, the properties of the panel can be returned to the condition before absorption of moisture by removal of moisture absorbed in the stimulable phosphor layer. Therefore, it has been desired to protect the stimulable phosphor layer from moisture reaching a surface of the stimulable phosphor layer.
In order to solve the above problems, the prior art has adopted a method in which a protective layer is provided on a surface of a stimulable phosphor layer on a support of a radiation image storage panel.
This protective layer is formed by, for example, a method in which a coating liquid for the protective layer is applied directly on the stimulable phosphor layer or a method in which a protective layer preliminarily formed separately is adhered onto the stimulable phosphor layer, as described in Japanese Unexamined Patent Publication No. 42500/1984.
Further, in Japanese Patent Application No. 18934/1985, the present inventors have proposed a method for forming a protective layer by applying on a stimulable phosphor layer a coating liquid for the protective layer containing regin material such as monomer, oligomer or polymer (hereinafter referred to as a radiation curring type resin or thermosetting resin) which may be polycondensated or crosslinked by irradiation of radiation and/or heating to be cured, and subsequently curing the regin material by irradiation of radiation and/or heating.
In order to accomplish the elongation of lifetime of a radiation image storage panel, further improvement, particularly, with respect to the humidity resistance, has been desired. However, in the present states, any method for reducing the water vapor permeability of a protective layer has been hardly studied.